Calender for treating both sides of a paper web

ABSTRACT

A calender for treating both sides of a paper web includes hard rollers and soft rollers. Working nips are formed between the juncture of each hard roller and soft roller. The roller stack can be loaded from one end, and preferably includes six to eight rollers. A changeover nip is formed by the juncture of two soft rollers or at the transition between two stacks. When using two stacks, each stack preferably has three to five rollers. At least one working nip has a dwell time of at least 0.1 ms. A heatable roller adjacent to the working nip is heated to a surface temperature of at least 100° C. The roller stack is loaded such that an average compressive stress in the working nip is greater than or equal to 42 N/mm 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a calender for treating both sides of apaper web. More specifically, the present invention relates to acalender that is suitable for manufacturing paper that can be used inphotogravure printing. The calender includes a roller stack that can beloaded from one end. The calender includes hard rollers and softrollers. Working nips are formed between the juncture of a hard rollerand a soft roller. The hard roller surface, disposed adjacent to theworking nip, can be heated.

2. Discussion of the Related Art

Calenders for treating both sides of a paper web are known, for example,from the 1994 brochure "Die neuen Superkalanderkonzepte" The NewSupercalender Concepts!, which is published by Sulzer Papertec company(identification number 05/94 d). These supercalenders are used for thefinal treatment of a paper web so that the web will obtain the desireddegree of smoothness, gloss, thickness, bulk, and the like. Thesesupercalenders are installed separately from an upstream paper machine.The soft or elastic rollers have an outer covering that is primarilymade of fibrous material. The heatable rollers are heated to a surfacetemperature of up to about 80° C. The average compressive stress in theworking nips during normal operation is between 15 and 30 N/mm².Additionally, in the lowest working nip, a compressive stress of 40N/mm² has been applied. The rollers are arranged in a roller stack. Theroller stack includes nine or ten rollers, which is sufficient for paperthat is to be simply finished, such as writing paper. Twelve to sixteenrollers are required for higher-quality papers, such as paper that issuitable for photogravure printing, technical papers, or compressionpapers. A calender for such high quality papers is expensive andrequires a large amount of space.

Compact calenders are also known. Compact calenders have a heatableroller, which forms a nip with a deflection-controllable soft roller.Two compact calenders can be connected in series to treat both sides ofa paper web. However, compact calenders can only be used to manufacturepapers that require simple finishing. These calenders can not be used totreat high quality papers, such as silicon based papers or papers forphotogravure printing. Compact calenders require that a large amount ofdeformation energy, in the form of heat, be added to operate thecalender. Therefore, the heatable rollers have a surface temperatureranging from 160° C. to 200° C. A great deal of heat energy radiatesfrom the compact calender, which must be exhausted using airconditioners. Because the roller diameter in a compact calender islarger (for sturdiness purposes) than the roller diameter in asuper-calender, higher loads per unit of length must be applied toproduce the required compressive stresses for the desired finishingresult. Furthermore, replacement rollers for the soft rollers areexpensive because they must also be deflection-controllable.

Accordingly, it is an object of the present invention to provide acalender that affords excellent finishing results, yet is smaller andless expensive to manufacture and operate.

SUMMARY OF THE INVENTION

The object is achieved in accordance with a preferred embodiment of thepresent invention by providing a calender for treating both sides of apaper web. The calender includes a plurality of hard rollers and aplurality of soft rollers that are aligned in a roller stack. The rollerstack has a first end and a second end. The stack includes a working nipformed by the juncture of a hard roller and a soft roller. At least oneof the plurality of hard and soft rollers includes a device for heatinga surface of the roller to a temperature of at least 100° C. The rollerstack is loaded from the first end such that the average compressivestress in at least one of the working nips is greater than or equal to42 N/mm². The at least one working nip has a predetermined width so thata dwell time of the paper web passing through the working nip is atleast 0.1 ms. The roller stack includes, in one embodiment, from six toeight rollers. A changeover nip is formed by the juncture of two softrollers. In a second embodiment the calender includes two roller stacks.Each of the first roller stack and the second roller stack has fromthree to five rollers.

The effect of the roller weight on the load per unit of length isdecreased by reducing the stack height. Therefore, in accordance withthe teachings of the present invention, it is possible to have the sameload per unit of length in the lowest working nip as compared to theprior art calenders, while the load per unit of length in the uppermostworking nip is greater than the load applied in supercalenders of theprior art. Surprisingly, it is therefore sufficient to only moderatelyincrease the deformation energy that is supplied, while still being ableto satisfactorily process high-quality papers. For example, heat can beadded at temperatures that are only slightly above the previouscustomary temperatures and, thus, only slightly increasing the heatradiation. In addition, many different heat transfer devices may be usedbecause the lower heat requirements of the present invention avoid thedifficulties encountered when using the high temperatures, which arerequired for a compact calender. The present invention also onlyrequires a relatively slight increase in the compressive stress appliedin the working nip, which can be mechanically tolerated withoutrequiring any structural modification of the calender assembly. At most,the soft roller covering material may need to be modified to accommodatethe slight increase in the heat and compressive stress.

Since both factors (increased heat and increased load) can be appliedsimultaneously in at least one working nip, preferably the lowestworking nip, unusually good results in the properties of the paper webafter final treatment can be achieved. This is true even when treatinghigh quality papers with a rapidly running calender. Because the rollerstack is not as tall as supercalenders of the prior art, lowerstructures can be built, which significantly reduce the installationcost.

The calender according to the present invention is preferably comprisedof a single roller stack of six to eight rollers or a double rollerstack of three to five rollers. Both the single roller stack and thedouble roller stack provide practically the same finishing results as acustomary twelve-roller calender that was previously considerednecessary to produce high quality papers that are suitable forphotogravure printing. Using two roller stacks has the additionaladvantage that the load per unit of length is less dependent on theweight of the rollers. Thus, a much higher load per unit of length canbe achieved in each of the uppermost working nips than was previouslythe case.

In a preferred embodiment, the dwell time of the paper web passingthrough a working nip is at most 0.9 ms. A surface of the rolleradjacent to the working nip is preferably designed to reach a maximumsurface temperature of 150° C. The roller stack is loaded so that anaverage compressive stress is less than or equal to 60 N/mm². Therefore,only a moderate increase in the surface temperature and the compressivestress is actually necessary as compared to conventional supercalenders.These slight increases can be tolerated because the increased valves areevenly distributed among the working nips.

The dwell time is preferably between 0.2 ms and 0.5 ms, the surfacetemperature is preferably between 110° C. to 125° C., and the averagecompressive stress is preferably between 45 N/mm² and 55 N/mm². It isparticularly advantageous for these requirements to apply to all or atleast a majority of the working nips.

The upper and/or lower rollers are preferably deflection controllablerollers. Thus, the compressive stress can be distributed evenly over theentire width of the rollers.

The upper and lower hard rollers are also preferably heated. Heat energyis preferably applied to the hard rollers because these rollers can bemore easily heated than soft rollers. This is especially true when theupper and lower rollers are deflection controllable, because thepressure fluid, which is used to adjust the deflection, can be heated tocontrol the heating of these rollers.

It is particularly beneficial for the soft rollers to have an outerplastic covering. Plastic covered rollers operate significantly betterthan rollers which are covered with a fibrous material at increasedaverage compressive stresses. The plastic covered rollers allowoperation at a compressive stress of more than 42 N/mm². In particular,the plastic covering should be designed to permit a compressive stressin the working nip of up to about 60 N/mm².

The plastic covering is preferably made of a fiber-reinforced epoxyresin, which typically has a useful life of at least 12 weeks.

In an additional embodiment of the present invention, the roller stackor stacks are arranged in-line (i.e., in series) with a paper machine ora coating machine. The paper web is thus at a relatively hightemperature at the intake nip of the calender (e.g., 60° C.) andtherefore the web only requires a slight addition of heat to providesufficient deformation. Plastic coverings, which are already desirablebecause of the higher compressive stresses that they can withstand, areparticularly suitable for in-line operations, because, in contrast withcoverings made of fibrous material, they are significantly lesssusceptible to marking. Therefore, plastic coverings rarely need to beremoved and reworked, for example, by grinding. Calenders comprised oftwo roller stacks have the additional advantage of being more suitablefor in-line operation, because the running paper web in each stack isfed through a lower number of working nips.

Each of the rollers in a roller stack is preferably driven independentlyof the other rollers. The paper web can therefore be independentlypulled in while the calender is running because all of the rollers canbe brought to the same speed before the nips are closed.

The roller stack is preferably covered by a protective hood whichreduces the amount of heat radiating from the calender. The protectivehood ensures that the manufacturing facility is not overheated, whichwould require excessive air conditioning. Conversely, the temperatureinside the hood is preferably maintained at a predetermined higher levelthan in conventional calenders, so that the addition of heat through theheating device can be minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,especially when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components, and wherein:

FIG. 1 is a schematic side view of a calender in accordance with thepresent invention;

FIG. 2 is a schematic side view of a second embodiment of the presentinvention; and

FIG. 3 is a schematic side view of a third embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a calender 1 having one roller stack isillustrated. The roller stack is preferably comprised of eight rollers.The eight rollers include a heatable deflection-controllable hard upperroller 2, a soft roller 3, a heatable hard roller 4, a soft roller 5, asoft roller 6, a heatable hard roller 7, a soft roller 8, and a heatabledeflection-controllable hard lower roller 9. This arrangement of theeight rollers creates six working nips 10, 11, 12, 13, 14 and 15 and achangeover nip 16. Each of the working nips 10-15 are formed by thejuncture of one hard roller and one soft roller. The changeover nip 16is formed by the juncture of two soft rollers 5 and 6. A web of paper 17is fed out of a paper machine or coating machine 18. The web 17 isguided by a plurality of guide rollers 19 so that it passes through theworking nips 10-12, the changeover nip 16, and the working nips 13-15.Thereafter, web 17 is wound onto a winding device 20. As the web 17passes through the top three working nips 10-12, only one side of thepaper web contacts the hard rollers 2, 4. However, as the web 17 passesthrough the three lowest working nips 13-15, only the opposite side ofthe paper web contacts the hard rollers 7, 9. Thus, the desired surfacestructure properties, such as smoothness and gloss, is produced on bothsides of the paper web.

The illustrated assembly is known in the art as an in-line operationbecause the output of the paper machine or coating machine 18 isdirectly connected to the input of the calender 1. In an in-lineoperation, each of the rollers 2-9 preferably is driven independently bya separate drive 21 so that the paper web 17 can be selectively pulledin during operation. Each of the soft rollers 3, 5, 6, and 8 has anouter covering 22 made of plastic. In a preferred embodiment, theplastic is a fiber-reinforced epoxy resin. This material is lesssusceptible to marking than a covering made of fibrous material. Thus,the soft roller has a significantly longer useful life, which isimportant for in-line operation. This material can also be subjected tohigher compressive stress and is resistant to higher temperatures than acovering made of fibrous material. This plastic covering is commerciallyavailable, for example, from the Scapa Kern Company of Wimpassing,Austria and is sold under the brand name "TopTec 4"™.

A control device 23 is operatively connected to the calender. Forexample, the force P with which the upper roller 2 is pressed downwardis controlled over a line 24. In a preferred embodiment, the lowerroller 9 is held stationary. However, the load can also move in theopposite direction, so that the force P acts on lower roller 9 and theupper roller 2 is fixed. The load determines the compressive stress thatis applied in the individual working nips 10-15. The compressive stressincreases from the top to the bottom because the weight of theindividual rollers is added to the loading force P. However, thedifferential increase in force in each stack according to the presentinvention is less than the differential increase in force in each stackof the prior art supercalenders which have from nine to sixteen rollers.

A deflection compensating device 27, 28 is disposed in each hard roller2, 9, respectively, to adjust the deflection of the upper roller 2 andthe lower roller 9, respectively. Control device 23 controls the amountof pressure that is applied along control lines 25, 26, via a pressuredevice, to the deflection compensating devices 27, 28, respectively, sothat the deflection in each roller 2, 9 is adjusted. Deflection devices27, 28 ensure that there is an even compressive stress applied over theaxial length of the roller. Any conventional deflection compensatingdevice can be used. However, it is preferred to use those devices inwhich support elements are arranged next to each other in a row, whichelements can be pressurized individually or in zones at differentpressures.

Hard rollers 2, 4, 7, and 9 are heatable, as shown by arrows H. Theamount of heat energy that is added is controlled by the control device23 along control lines 27a, 28a, 29, 30. The heating may be effected,for example, by electric heating, radiant heating or a heat exchangemedium. A protective hood 31 provides heat insulation and ensures thatheat that is radiated as a result of the heating is exhausted into theenvironment to only a slight extent.

The average compressive stress σ applied in at least the lowest workingnip 15, and preferably in all of the working nips 10-15, is preferablymaintained between 45 N/mm² and 60 N/mm² due to force P. The surfacetemperature of the heatable rollers 2, 4, 7, and 9 is preferablymaintained between 100° C. and 150° C. due to the heating H. Thediameter of the rollers and the elasticity of the covering 22 areselected so that a nip width of about 2-15 mm, and preferably about 8mm, is maintained. The dwell times t of the web 17, in each working nipis about 0.1 to 0.9 ms. The dwell time is a function of the web speed.In a preferred embodiment, the temperature T is only slightly above thelower limit, for example 110° C., and the compressive stress is onlyslightly above the lower limit, for example 50 N/mm².

The present inventors have determined that the printability of naturaland lightly coated papers is not necessarily related to the gloss orsmoothness achieved in the paper web, but is instead related tocompression or its reciprocal value bulk (in cm³ /g). The measurement ofprintability in photogravure printing is determined by the number of"missing dots" in the quartertone and halftone area. The best results inthat regard are thus obtained when it is ensured that all of the limitsspecified above are maintained in all working nips.

FIG. 2 shows a two roller stack calender 32, where each stack has fiverollers. Thus, the calender is known as a 2×5 roller calender 32. Thefirst stack includes a hard upper roller 33, a soft roller 34, a hardroller 35, a soft roller 36, and a hard lower roller 37. The secondstack includes a hard upper roller 38, a soft roller 39, a hard roller40, a soft roller 41, and a hard lower roller 42. Each stack thereforehas three working nips through which the paper web 43 runs in such a waythat in the first stack one surface of the web comes into contact withthe three hard rollers and in the second stack the other web surfacecomes into contact with the three hard rollers. The heating of therollers, the deflection control of the upper and lower rollers, and theloading of the two roller stacks can be achieved in a similar manner tothat of the calender illustrated in FIG. 1.

FIG. 3 shows a one roller stack calender 44, which stack has sixrollers. The single stack includes a hard upper roller 45, a soft roller46, a hard roller 47, soft rollers 48 and 49, and a hard lower roller50. A changeover nip 51 is located between the soft rollers 48 and 49.One surface of the paper web 52 contacts hard rollers 45, 47 and theother web surface contacts hard roller 50. Thus, one surface of thepaper web 52 is finished above the changeover nip 51, while the othersurface is finished below nip 51.

The results of paper treatment can often be improved when the rollers,particularly the middle rollers, are held by levers (not shown) so thatthe overhanging weights are preferably compensated for by supportdevices, as is known from European reference EP 0 285 942 B1.

Having described the presently preferred exemplary embodiment of acalender for treating both sides of a paper web in accordance with thepresent invention, it is believed that other modifications, variationsand changes will be suggested to those skilled in the art in view of theteachings set forth herein. It is, therefore, to be understood that allsuch modifications, variations, and changes are believed to fall withinthe scope of the present invention as defined by the appended claims.

We claim:
 1. A calender for treating both sides of a paper web,comprising:a plurality of hard rollers and a plurality of soft rollersbeing aligned in a roller stack, said roller stack having a first endand a second end, said stack including a plurality of working nips eachbeing formed by the juncture of one of said hard rollers and one of saidsoft rollers, at least one of said plurality of hard and soft rollersincluding means for heating a surface of said roller to a temperature ofat least 100° C., said roller stack being loaded from said first endsuch that the average compressive stress in at least one of said workingnips is no less than 42 N/mm², said at least one working nip having apredetermined width so that a dwell time of said paper web passingthrough said working nip is at least 0.1 ms.
 2. The calender accordingto claim 1, wherein said dwell time is at most 0.9 ms, said heatingmeans heats said roller surface to a maximum temperature of 150° C.,said roller stack being loaded such that said average compressive stressis at most 60 N/mm².
 3. The calender according to claim 2, wherein saiddwell time ranges from 0.2 ms to 0.5 ms, said surface temperature rangesfrom 110° C. to 125° C., and said average compressive stress ranges from45 N/mm² to 55 N/mm².
 4. The calender according to claim 3, wherein saiddwell time ranges, said surface temperature ranges and said averagecompressive stress ranges apply to a majority of said working nips. 5.The calender according to claim 1, wherein said roller disposed at saidfirst end and said roller disposed at said second end aredeflection-controllable.
 6. The calender according to claim 5, whereinsaid deflection controllable rollers are heatable.
 7. The calenderaccording to claim 6, wherein said soft rollers include a plasticcovering.
 8. The calender according to claim 7, wherein said plasticcovering supports a compressive stress up to 60 N/mm².
 9. The calenderaccording to claim 7, wherein said plastic covering is substantiallycomprised of a fiber-reinforced epoxy resin.
 10. The calender accordingto claim 1, wherein said roller stack is arranged in-line with one of apaper machine and a coating machine.
 11. The calender according to claim1, wherein each of said plurality of hard and soft rollers are drivenindependently.
 12. The calender according to claim 1, wherein saidroller stack is covered by a protective hood that reduces heat radiationemitting from said roller stack.
 13. The calender according to claim 1,wherein the roller stack includes from six to eight rollers, achangeover nip being formed by the juncture of two soft rollers.
 14. Thecalender according to claim 1, further comprising a second roller stack,each of said first roller stack and said second roller stack having fromthree to five rollers.